Solubilization of Membranes by Styrene Maleic Acid (SMA) Results in Formation of Nanodiscs with Retention of Native Lipid Composition

Abstract

Purification of membrane proteins has been a real headache for the scientific community during the last decades. Recently, a new purification method has emerged using the SMA (styrene-maleic acid) co-polymer, which is able to solubilize membranes into small bilayer discs. Its importance lies mostly in the presumed ability of retaining the native lipid environment of the membrane proteins, preventing misfolding and/or loss of activity. However, it cannot be excluded that SMA preferentially solubilizes specific lipids, thereby modifying the extent to which the native lipid environment is indeed retained.To find out whether preferential solubilization by the polymer occurs, we performed an extensive biophysical study using model membranes of synthetic lipids with well-defined compositions. Solubilized and non-solubilized material was obtained by adding limiting amounts of SMA polymer to the phospholipid vesicles. The mixture was centrifuged and lipid analysis was performed on the supernatant, containing the nanodiscs, and the non-solubilized fraction in the pellet. This procedure was performed for mixtures of different phospholipids, in which we varied the acyl chain length, unsaturation, head group charge and lipid phase. The results showed no preferential solubilization in any of these lipid mixtures, the only exception being a raft-like system (DOPC/SM/cholesterol), where it was shown that SMA is incapable of solubilizing SM/cholesterol-rich domains into nanodiscs.The results highlight the potential of SMA polymer as a strong alternative for membrane protein purification, in which the native membrane lipid composition is generally retained. Interestingly, the behavior of the raft-like systems upon addition of SMA resembles the behavior of detergent-resistant membranes (DRMs) when membranes are treated with detergent, paving the way for novel approaches of lipid raft isolation.